Diaphragm carburetors generally have a flexible diaphragm disposed in a fuel chamber which opens to main and idling jets or orifices. Typically, the diaphragm divides the chamber into a wet chamber side which is supplied with fuel and is subjected to sub-atmospheric pressure during operation of the engine and a dry chamber side which may be subjected to atmospheric pressure. The diaphragm controls a fuel inlet valve disposed between a supply of fuel and the wet chamber side. As the engine draws fuel from the wet chamber side, the quantity of the fuel in the wet chamber will decrease, and the diaphragm will move against the bias of a spring to open the needle valve and allow fuel to enter the chamber.
In operation, the diaphragm repeatedly opens and closes the inlet needle valve so that fuel can enter the diaphragm chamber in response to sub-atmospheric pressure in the throat of the carburetor. Accordingly, a certain amount of fuel can be maintained in the diaphragm chamber at a substantially constant pressure to supply fuel to the main and idling jets. The diaphragm must be extremely flexible because the valve must open and close rapidly with a small pressure differential which is typically one to two inches of water. It must also operate over a wide temperature range of about -40.degree. F. to 180.degree. F.
For over 40 years the small engine industry has experienced many problems in the manufacture, in service use, and performance of fuel metering diaphragms. Currently most diaphragms are made of a rubber coated silk material. In use, the performance characteristics of these diaphragms change and deteriorate substantially. The inventors have discovered that the silk fibers absorb moisture from the atmosphere and/or the fuel, resulting in a diaphragm that changes its performance characteristics depending on the ambient weather conditions, such as temperature and humidity, and the moisture content of the fuel which severely limits the performance of the fuel metering diaphragms. Other materials, such as woven nylon, have also been found to be ineffective because they produce a diaphragm which is too thick and/or inflexible to adequately respond to small pressure changes. When in prolonged contact with liquid fuel, the coating also swells or increases in volume and deteriorates in hardness, tensile strength and ultimate elongation.
Additionally, currently available manufacturing processes are unable to produce diaphragms having the dimensional tolerances and performance characteristics required. A two-ply rubber coating is used with one ply or layer being applied to both sides of a sheet of material from which the diaphragm is cut. This increases the rigidity of the material which, when coupled with the above mentioned problems of the material, results in a diaphragm that in incapable of consistently performing as required.
Previously, the silk fabric with an uncured rubber coating on both sides was heated in an oven to fully cure the rubber and then cooled to room temperature. Thereafter, a stack or pile of several of the resulting flat composite sheets were placed in a multiple cavity mold and simultaneously molded under a force of 6-20 tons for a 54 cavity mold at a temperature of 330.degree. to 375.degree. F. for 4 to 8 minutes to form a bellows or convolution in the diaphragm. Thereafter, the molded sheets were cooled and trimmed in a die and press to cut or blank out the individual diaphragms from each sheet. When in service in a carburetor, the convolution tends to become smaller, diminish or even disappear and thus is not permanent and degrades performance of the diaphragm.
Since up to four cured two ply rubber coated sheets are molded at the same time to form the bellows, they are subjected to an uneven application of pressure and heat. This results in different performance properties for each diaphragm.